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The acromioclavicular (AC) joint cyst is a rare sequela of complete supraspinatus tendon rupture and arthritis of the AC joint. It appears as a swelling on the AC joint and must be distinguished from tumours of the shoulder and neck region. Besides physical examination, magnetic resonance imaging is the most useful tool for correct diagnosis. As reported in the literature, therapeutic procedures include observation, aspiration, excision of the cyst with or without resection of the AC joint, and repair of the underlying supraspinatus tendon rupture. All procedures should be based on consideration of the comorbidity and recurrence of the cyst. Combinations of cyst excision and rotator cuff repair or AC resection seem to be the most promising therapeutic options. We report on a 75-year-old patient with a painless swelling over the AC joint who had suffered shoulder trauma 15 years previously.